Document config variables and values

[dhrdlicka]

Is there any references/documentations for Linux users to modify the configuration files? The documentation here doesn't list any details, only leads the users to the GUI which Linux does not have for now. I don't know, for example, available cpu_family, or options not present in auto-generated 86box.cfg.

Originally posted by @OliverLew in 86Box/86Box#1962 (comment)

[jriwanek]

There may be some additional options we could document, but 86Box on Linux now has a UI.

[ReimuNotMoe]

In case someone is searching on the Internet about how to enable display resizing in Linux: add vid_resize = 1 in General section.